The invention is applicable to telephone systems which are known as "Intelligent Networks" (IN) or "Advanced Intelligent Networks" (AIN) and which employ out-of-band signalling systems such as Signalling System No. 7 (SS7), also known as Common Channel Signalling No. 7 (CCS7). Such signalling systems exchange Transaction Capabilities Application Part (TCAP) messages or queries between network node switching elements to set up and route calls. The TCAP and ISUP messages are handled by a data communications system separate from the trunks which carry the calls themselves. The elements of such intelligent telephone systems include Service Switching Points (SSPs) interconnected by trunks for carrying calls, and Signal Transfer Points (STPs) and Service Control Points (SCPs) connected to each other and the SSPs by SS7 data links for carrying signalling. A Service Control Point is an "intelligence centre" with access to application databases enabling it to deliver various combinations of features, such as 1-800 number service and call redirection. A Signal Transfer Point (STP) is a signalling hub or concentrator, typically connecting several Service Switching Points to an SCP. A Service Switching Point (SSP) is a network node normally associated with a central office switch equipped with Signalling System Number 7 (SS7) messaging links to permit communication with the SCPs and which has special AIN call processing software including a plurality of "Point-in-Call triggers" which can be provisioned or set to interrupt call processing momentarily and initiate a TCAP query to the SCP for instructions on how to complete the call processing. Based upon the instructions, the originating end office seizes a trunk to a neighbouring switch and creates an Initial Address Message which it sends to the neighbouring switch via the signalling network. The Initial Address Message includes various parameters which will control routing of the call to its destination.
In circuit-switched telephone systems, each subscriber has a unique public directory number which is associated with a particular circuit of a switch at the subscriber's local office. As a general rule, if the subscriber transfers to a new service provider, the subscriber is assigned a new public directory number. For various reasons, it is desirable for a subscriber to be able to change service provider while keeping the same public directory number and any specific service features associated with it.
Various "local number portability" schemes have been proposed for "porting" a subscriber's directory number to a different switch and routing calls to it automatically, so that other subscribers may dial the same public directory number and still reach the subscriber, without knowing that the ported number is at a different switch. Procedures for achieving such "local number portability" were promulgated in a document entitled "Generic Switching and Signalling Requirements for Number Portability", Illinois Commerce Commission Workshop on LNP, Illinois, April 1996.
The former document proposes that, when a directory number is ported to a different switch, an entry will be made in the SCP database linking the ported directory number with the Location Routing Number of the recipient switch, i.e. the switch to which the subscriber transferred. Each originating end office will be notified that a number (NPA-NXX) from the "donor" office had been ported and will set a trigger to query the SCP for instructions when trying to route any subsequent calls to that NPA-NXX. It is preferred to set a trigger which will be actuated by any directory numbers beginning with that NPA-NXX rather than set a trigger for each individual ported directory number. The SCP will return the Location Routing Number of the recipient switch. The originating switch will incorporate the Location Routing Number and the original dialled digits into a special Initial Address Message and route the call to the recipient switch, which will complete the call to the "ported" subscriber. The Initial Address Message is termed "special" because it will have the Location Routing Number in place of the usual "Called party number" and the original dialled digits (called number) as a parameter called the "Generic Address Parameter". This Generic Address Parameter is defined in Bellcore GR-317-CORE. At the recipient end office switch, the presence of the Location Routing Number (which cannot be dialled by a subscriber) in place of the called number will prompt the switch to look to the Generic Address Parameter for the called number to which the call should be completed. Such an arrangement entails modifications to the SCP software, which is not particularly expensive because the SCPs are relatively few in number. A major disadvantage of this procedure, however, is that it requires every SSP in the network to be modified to give it the capability of creating the special Initial Address Messages and understanding those it receives. This entails considerable expense since there might be several hundred SSPs in the network.